Batteries can be used to store the electricity since there may be times the solar panels will be blocked from the Sun by objects or the dark side of celestial bodies. For extended burns in the darkness the fuel cells happen to be a good choice. When the power is provided by the fuel cells the majority of the mass flow (about 69.2%) is liquid fuel and oxidizer used by the fuel cell. Thus the ion engine powered by the fuel cell may be seen as having much more modest but still impressive effective Isp of 1293 sec. However if the burn doesn't take more than a couple of hours the stack of RTGs providing the same amount of power (and thus thrust) tend to be heavier than the fuel cell array and its fuel tank. The RTGs should be reserved for very long low-thrust burns in the deep space.

The ion engine is good for fine tuning of orbits. It was also a popular propulsion method for planes on planets on which jet engines don't work, though its current thrust falloff reduces the value there. Due to its great fuel efficiency it is also well-suited for interplanetary travel, but maneuvers tend to take a long time to complete due to its very low thrust-to-weight ratio -- it is advised to use it for very small craft, and to use physics-warp while propelling with it. Usually the engine is used on long range craft due to its high efficiency. But when less delta-v is required, overall, it can be easily surpassed by smaller liquid fueled engines such as the 48-7S "Spark" engine with a lot better TWR.

It is impossible to build an ion-rocket which can defeat gravity on Kerbin, because the engine isn't even strong enough to lift itself against gravity, let alone itself and its fuel, a battery and a probe core. But when on a low-gravity moon like Minmus or Gilly it is possible to land, start, enter orbit and reach escape velocity with ion-propulsion alone. Since 0.23.5, it is technically possible to create an ion-powered probe, albeit with a minimum of parts, which will be able to defy Mun gravity. With that in mind, it is also possible to resist gravity with lightweight Ion craft on Duna, Moho, Dres, Eeloo, and every in-game moon with two exceptions: Laythe and Tylo have too high gravity for a single ion thruster, xenon tank, probe core, and battery.

While it is possible to build an airplane powered solely by this engine, the ion engine's efficiency is awful in the atmosphere. Unless you are going very far from the KSC, jet planes are much cheaper and more efficient.

Ion-powered "ferries" may also be useful for moving fuel, oxidizer and/or kerbonauts between two larger vessels, by keeping the large craft at such range that only one of them is within draw distance from the "ferry" at any moment, performance loss can be avoided. It is generally more fuel-efficient to move fuel and oxidizer between two ships using a ferry than it would be to dock the larger craft together using their own engines and RCS.

Because it uses only about 0.485 units of xenon per second, one PB-X50R Xenon Container with 400 units of xenon can supply the engine for almost 14 minutes. The other larger tank PB-X150 Xenon Container with 700 units of xenon has enough to supply the engine more than 24 minutes.

Electrical vs Xenon consumption

While the consumption ratios listed in the part.cfg files are normally relative mass flows (1.8 electricity and 0.1 xenon here), this breaks down somewhat with massless resources like electricity. Rather, the entire mass flow goes to the xenon, with the relative ratio (1.8 to 0.1, or 18 in total) creating a seemingly disproportionate drain.

Product description

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By emitting ionized xenon gas through a small thruster port, Dawn can produce incredibly efficient propulsion, but with a downside of very low thrust and high energy usage. According to ISP Electronics sales reps, the rumours of this engine being powered by "dark magic" are largely exaggerated.

— Ionic Symphonic Protonic Electronics

”

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By emitting ionized xenon gas through a small thruster port, the PB-ION can produce incredibly efficient propulsion, but with a downside of very low thrust and high expense. The general perception of this engine as being powered by "witchcraft" has unfortunately given it a sour reputation.

— Ionic Protonic Electronics (0.18-0.23)

”

Trivia

Although the 2 kN IX-6315 "Dawn" Electric Propulsion System is considered to have very low thrust in KSP, real-life Hall effect thrusters typically have orders of magnitude less thrust usually below 1 N (0.001 kN). They make up for this by having a service life of thousands of hours of continuous operation and consuming fuel extremely slowly, which would have been impractical in the game.

The name "Dawn" is a reference to NASA's Dawn spacecraft, which is NASA's first interplanetary space probe to use an ion engine for propulsion.